CFD simulations of two-phase flow

When doing computational fluid dynamics (CFD), there are two methods generally used to describe two-phase flow, which are Euler-Euler (E-E) model and Euler-Lagrange (E-L) model. In E-E model, particles are treated as a fluid, and also have stress, pressure and other characters that possessed by a continuous phase. On the contrary, E-L model treats particles as a discrete phase, and tracks movement of individual particles. In this chapter, both E-E and E-L methods are used and compared in the simulations of coal combustion, pneumatic conveying and aeolian sand movement. E-L model is a very time-consuming method, but it avoids extra models for particle phase. E-E model directly gives the statistical regularity of particle performances, but unreasonable sub-models may result in unreasonable predictions. This chapter analyzes advantages and disadvantages of both E-E and E-L models and proposes a new methodology that combines two methods to make reasonable predictions while spending comparative less computational time

PDPA studies on turbulence and particle concentration in a swirl burner

To develop low-pollutant burners, the effect of inlet blocking on NO formation is studied. It is recognized that the NO formation is affected by the turbulence, coal concentration and temperature. In this paper, the turbulence and coal concentration in swirl burners with an inlet blocking or a coal concentrator were measured. A DANTEC-made 3-D phase-Doppler particle anemometer (PDPA) installed in the State key Lab. of Clean Coal Combustion is used to measure the particle velocity and concentration. The measurement results are compared with those obtained by numerical simulation. Both measurement and simulation results indicate that either the blocking or the coal concentrator increases the turbulent kinetic energy at the inlet region and the particle concentration in the near-axis region, which is favorable for reducing the NO emission during coal combustion

Simulation of swirling coal combustion using a full two-fluid model and an AUSM turbulence-chemistry model

A full two-fluid model of reacting gas-particle flows with an algebraic unified second-order moment turbulence-chemistry model for the turbulent reaction rate of NO formation are used to simulate swirling coal combustion. The sub-models are the k–ε–kp two-phase turbulence model, the EBU–Arrhenius volatile and CO combustion model, the six-flux radiation model, coal devolatilization model and char combustion model. The prediction results are in good agreement with the experimental results taken from references

Investigation of intracranial aneurysm hemodynamics following flow diverter stent treatment

Flow diverters (FDs) are high density meshed stents designed to reduce blood flow into intra-cranial aneurysms. Though the FD is one of many intracranial aneurysm (IA) treatments, FD implantation may also result in the growth and rupture of residual aneurysms. The purpose of this research is to investigate the effect of FD implantation on IA hemodynamics. Computational fluid dynamics (CFD) was conducted to analyze dynamic and resistance forces after FD deployment. Simulation results for the successful case (patient A) showed that FD flow resistance force was higher than dynamic force. This indicated that the FD provided sufficient resistance to reduce flow into the aneurysm. As a result, flow velocity magnitude at the aneurysm neck was reduced by 95%. On the other hand, the flow velocity magnitude at the aneurysm neck was reduced by about 50% for the unsuccessful case (patient B). The reason was that the flow resistance force at the aneurysm neck section was calculated to be lower than the flow driving force. In order to completely occlude the aneurysm, a higher resistance FD stent is to be required to suppress the dynamic forces. Patient-specific hemodynamic simulations offer means of quantitative estimation FD treatment outcomes

Using computational fluid dynamics to predict clinical outcome of aneurysms treated by flow diverters

Purpose: The haemodynamic profiles of aneurysms treated by flow diverters (FD) were analysed with computational fluid dynamics (CFD) to determine the profiles associated with successful and failed treatments. Materials & Methods: Using ANSYS software, patient-specific CFD were used to simulate haemodynamic profiles including the presence of jet flow, energy loss, volume flow and wall shear stress in 4 successful occlusions of aneurysms and 4 failed cases after FD deployment. A successful case was defined as occlusion within 6 months. Failed cases include rupture post treatment and failure to occlude within 6 months. In the 4 failed cases, haemodynamic profiles were examined again after a hypothetical second intervention. This involved replacing the failed FD with a hypothetical optimally deployed FD or simulated placement of a second FD within the first. Results: Where successful occlusions were achieved, a marked obliteration of jet flow was observed. Flow entering the aneurysm sac was diverted via the centre of the FD and exited smoothly into the parent arteries. These observations were supplemented by a reduction in the other haemodynamic profiles. In failed cases, the strong jet flow persisted without reduction of the other parameters. Aneurysm neck geometry significantly influenced the efficacy of the FD. Conclusion: Haemodynamic indices, as calculated using CFD techniques, have close correlation with FD treatment outcome. CFD could be potentially useful as a planning tool for neurointerventionists by simulating an optimised flow diverter deployment strategy prior to the procedure and evaluating post-treatment outcome

Numerical simulation of NOx formation in coal combustion with inlet natural gas burning

A full two-fluid model of reacting gas-particle flows and coal combustion is used to simulate coal combustion with and without inlet natural gas added in the inlet. The simulation results for the case without natural gas burning is in fair agreement with the experimental results reported in references. The simulation results of different natural gas adding positions indicate that the natural gas burning can form lean oxygen combustion environment at the combustor inlet region and the NOx concentration is reduced. The same result can be obtained from chemical equilibrium analysis

Statistical particle stress in aeolian sand movement-derivation and validation

The statistical particle stress (SPS) is a result of group particle movement, which is not referred directly from E–L (Euler–Lagrange) calculations. This paper derives SPS in Eulerian regime and proves that in aeolian sand movement, as the height increases the gas stress also increases while the SPS decreases; however, the sum of gas stress and SPS keeps to be a constant value, which equals to the gas stress in particle absent region. This paper also suggests that SPS predominates in the momentum transportation of particle phase

Studies of the effect of a coal concentrator on NO formation in swirling coal combustion

To develop low-pollution burners, the effect of a coal concentrator on NO formation in swirling coal combustion is studied using both numerical simulation and experiments. The isothermal gas–particle two-phase velocities and particle concentration in a cold model of swirl burners with and without coal concentrators were measured using the phase Doppler particle anemometer (PDPA). A full two-fluid model of reacting gas–particle flows and coal combustion with an algebraic unified second-order moment (AUSM) turbulence-chemistry model for the turbulent reaction rate of NO formation are used to simulate swirling coal combustion and NO formation with different coal concentrators. The results give the turbulent kinetic energy, particle concentration, temperature and NO concentration in cases of with and without coal concentrators. The predicted results for cold two-phase flows are in good agreement with the PDPA measurement results, showing that the coal concentrator increases the turbulence and particle concentration in the recirculation zone. The combustion modeling results indicate that although the coal concentrator increases the turbulence and combustion temperature, but still can remarkably reduce the NO formation due to creating high coal concentration in the recirculation zone

Experimental and numerical research on the particles movement in the internally circulating fluidized bed

The experimental and numerical research in a cold model of internally circulating fluidized bed(ICFB) with "∧" type of air distributor was conducted to analyze the characteristics of the particles movement.And the obtained two kinds of results were compared for various operating conditions.Results of numerical simulation show that the large scale circulating flow in the bed is formed,similar with the phenomena in experiments.The lateral movement of particles may become very strong in the top and bottom of the bed.And the particle diffusion in the bed with uneven air distributions is better than that with even air distributions

Propose a wall shear stress divergence to estimate the risks of intracranial aneurysm rupture

Although wall shear stress (WSS) has long been considered a critical indicator of intracranial aneurysm rupture, there is still no definite conclusion as towhether a high or a lowWSS results in aneurysm rupture.The reasonmay be that the effect ofWSS direction has not been fully considered. The objectives of this study are to investigate the magnitude of WSS (|WSS|) and its divergence on the aneurysm surface and to test the significance of both in relation to the aneurysm rupture. Patient-specific computational fluid dynamics (CFD) was used to compute WSS and wall shear stress divergence (WSSD) on the aneurysm surface for nineteen patients. Our results revealed that if high |WSS| is stretching aneurysm luminal surface, and the stretching region is concentrated, the aneurysm is under a high risk of rupture. It seems that, by considering both direction and magnitude ofWSS,WSSD may be a better indicator for the risk estimation of aneurysm rupture (154)